But expecting students to use only the grammar they learn at home (and that seems like the case you're making) makes about as much sense as expecting students to use only the math they learn at home.
I expect students to learn the grammar of standard English if they are to make it through school and university, but then I am a teacher as well as a writer. I don't suggest that one's native dialect, (which it is important to stress has a perfectly good and learnable, yet different from the standard's, grammar), is the one to use in writing an essay or one's thesis. Rather than trying to coerce a person, who only speaks the dialect they learned at home, with imprecise statements about their grammar being incorrect and/or imprecise, you would be going far to point out to them, that they need to learn to write, and to a lesser degree to read, standard (US / UK) English. There's a vast difference between telling somebody that they have no grammar, and telling them that they have to, in effect, learn a new language, with different grammatical rules. And they have to learn when and where to use their different languages / dialects / registers. That's all I'm saying. Many people are incabable of telling a coherent story, but that has little to do with their grammar, but their story-telling skills. This is the problem I have with most grammar mavens, that while I've spent a goodly part of my academic and post-academic life reading about languages and linguistics, they cannot be bothered with learning a bit more about language and linguistics, than what is presented in their old-fashioned and out-dated manuals of usage. That's all I'm saying.
